This invention relates generally to treatment of wastewater in systems employing recycled activated sludge.
The activated sludge process has been used for many years for the removal of biological oxygen demand (BOD) from wastewater. This process involves, in general, mixing the wastewater with recycled sludge containing microorganisms (biomass) and exposing the mixed liquor to aeration under conditions such that the biological oxygen demand of the wastewater is metabolized. Following such treatment, the mixed liquor is introduced into a settling tank, termed a secondary clarifier, in which the biomass settles out and concentrates while the clear liquor is discharged overhead for return to receiving streams or reservoirs, with or without further treatment. A major portion of the biomass which is concentrated and settled at the bottom of the clarifier, is recycled to the treating system for admixture with wastewater influent and a minor portion purged to maintain a constant biosolids inventory within the system.
The activated sludge process has been extensively described in the literature and several of its modifications are summarized in a special report on Wastewater Treatment by R. H. Marks in the June 1967 issue of POWER.
Among the problems encountered in operation of the activated sludge process was the difficulty, at times, in effecting desired separation of the biomass from the treated wastewater in the secondary clarifier. One of the reasons for the poor settling properties of the treated wastewater was found to be the proliferation of high surface area or filamentous species of microorganisms resulting in a phenomenon known as "bulking". Various modifications of the basic activated sludge process were proposed to overcome the bulking problem as described by Spector in the introductory paragraphs of U.S. Pat. No. 4,056,465.
As stated in U.S. Pat. No. 4,056,465, while certain of the methods theretofore proposed were effective in avoiding bulking biomass, they were not capable of doing so under conditions which also were effective in removal of nitrogen and/or phosphorus nutrient values from the wastewater influent. The method disclosed in this patent favors the selective production of nonbulking biomass while also obtaining effective removal of phosphates. By the proposed method of the patent, the wastewater and recycled sludge are initially admixed under anaerobic conditions in the substantial absence of oxygen or oxidizing agents and subsequently subjected to aeration followed by clarification. By a further alternative modification disclosed in the patent, nitrates and/or nitrites (NOX) are also removed by interposing an "anoxic" zone between the anaerobic zone and the aeration zone.
Thus, while the problems associated with bulking sludge have been largely overcome, there remains another major problem in sludge separation from biologically treated wastewater. This problem is the observed phenomenon of "rising sludge"; that is the ascent to the surface or upper regions of the secondary clarifier of sludge which had originally settled to the bottom of the clarifier. This phenomenon is different from the problem of filamentous sludge which does not settle adequately in the clarifier.
Rising sludge can occur in any activated sludge system but is particularly evident when the effluent from the aeration zone entering the clarifier, contains NOX, which is subsequently reduced to elemental nitrogen gas by the oxygen demand of the sludge at the bottom of the clarifier. When the concentration of dissolved nitrogen is in excess of that in equilibrium with nitrogen at the pressure existing in the sludge in the clarifier, excess nitrogen degasses; this results in formation of bubbles within the biomass and some or all of the settling biomass floats with the aid of gas bubbles to the top of the clarifier where it is discharged from the system with the effluent wastewater. This discharge of biomass results in excessive solids concentration in the treated water and diminished ability to maintain an inventory of biological solids within the system. No methods have heretofore been developed which effectively address this problem.
There are a variety of designs for clarifiers and for the introduction of mixed liquor to these clarifiers. In a typical system the mixed liquor from the aeration zone is discharged into an open channel or trough through which it flows and discharges into a receiving basin or so-called "effluent pit". The mixed liquor is withdrawn from the bottom of the pit through an effluent pipe, flowing downwardly for some distance through the pipe then laterally to an upwardly directed leg extending from the end of the lateral leg of the pipe through the bottom of the clarifier to a discharge level near the top of the clarifier. By the described arrangement, the height of the liquid in the clarifier is substantially the same as that in the effluent pit. This conventional arrangement is illustrated in FIG. 1 of the accompanying drawings. Certain of these systems also provide means to aerate the mixed liquor, via air sparging, during passage from the aeration zone to the effluent pit and in the effluent pit itself.
In an investigative study undertaken to determine the cause for the rising of bubbles in the clarifier, which led to the present invention, it was determined that these bubbles were formed at least in part as a result of dissolution of nitrogen from the air entrained in the mixed liquor and a concomitant rise in dissolved nitrogen above that in equilibrium with ambient air during its transport from the aeration basin to the clarifier.